1. Field of the Invention
The present invention is directed to processes for the production of normally gaseous mono- and di-olefins, particularly ethylene, propylene and butadiene, by thermally cracking a hydrocarbon feedstock in the presence of steam at elevated temperatures which involves introducing a hydrogen donor material, such as hydrotreated steam cracked tar oils, into a stream of steam cracked effluent at or downstream of the point where the furnace effluent reactions are quenched so as to prevent thermal degradation reactions of the steam cracked liquids.
2. Discussion of Background and Material Information
The use of hydrogen donor chemistry to in some manner alter or control the thermal conversion of hydrocarbon oils is known in the art. For example, U.S. Pat. Nos. 2,953,513 and 2,873,245, commonly owned with the present application, issued in 1959 and 1960, are directed to the concept of hydrogen donor diluent cracking (HDDC). In such processes, hydrogen donor oils, which are generally hydrotreated aromatic oils, are used to control and/or enhance the thermal cracking of heavy hydrogen deficient oils such as residua.
U.S. Pat. No. 4,284,139, SWEANY, is directed to a process for upgrading the oil production from a heavy oil reservoir by contacting the heavy oil with a hydrogen donor diluent and subjecting the mixture to thermal cracking in a hydrogen donor diluent furnace. The disclosed purpose for doing so is to break down the heavy molecules which already exist in naturally occurring heavy oils. Thus, SWEANY uses a variation of the conventional HDDD process to enhance the stimulation and upgrading of oil production from heavy oil reserves.
U.S. Pat. No. 4,430,197, POYNOR et al., is directed to a hydrogen donor diluent cracking process in which heavy hydrocarbonaceous material is thermally cracked in a cracking coil in the presence of a hydrogen donor solvent. POYNOR et al., therefore, also uses a variation of a conventional HDDC process, which involves heat soaking, in the presence of a hydrogen donor, pitch obtained from the HDDC process. This heat-soaked pitch is then recycled and cracked in the hydrogen donor diluent process.
U.S. Pat. No. 4,397,830, UEMURA et al., is directed to a process for producing carbon fibers which involves heat treating a feed stock pitch by mixing 100 parts by volume of a heavy fraction oil boiling not lower than 200.degree. C. obtained by steam cracking petroleum with 10 to 200 parts by volume of a hydrogenated oil selected from a group consisting of aromatic nucleus hydrogenated hydrocarbons of appropriate carbon ring number and/or boiling range including hydrogenated cat cracked oil.
U.S. Pat. No. 4,596,652, SHIBATANI et al., is directed to a process for producing a mesophase pitch for carbon filter production, which involves pretreating the raw pitch material at elevated temperature under a pressurized hydrogen atmosphere followed by heat treating the pitch at 350.degree. C. to 550.degree. C. while supplying the pitch with a hydrogen donor.
UEMURA et al. and SHIBATANI et al. both teach the use of hydrogen donors to control or modify the heat soaking of pitches to produce preferred feeds for the production of carbon fibers. In this regard, these references disclose that the hydrogen donors mitigate the formation of quinoline insolubles during heat soaking of the starting pitch. Quinoline insolubles are undesirable for carbon fiber production and are conventionally classified as higher molecular weight asphaltenes or coke.
U.S. Pat. No. 3,755,143, HOSOI et al., teach the pyrolysis of crude oil or fractions thereof, followed by desulfurization by hydrogenation of the polycyclic aromatic tar produced in the pyrolysis reaction followed by alkylation or hydrogenation of the resultant product using the hydrogen produced in the pyrolysis reaction. Thus, HOSOI et al. disclose the hydrogenation of SCT to produce an improved product using conventional catalysis to accomplish their hydrogenation step.
U.S. Pat. No. 4,260,474, WERNICKEet al., relate to thermal cracking of heavy fractions of hydrocarbon hydrogenates. The disclosed process involves hydrogenation of VGO at a temperature of about 340.degree. C. and subsequent recovery of a hydrogenated VGO boiling above about 340.degree. C. which is then steam cracked to produce naptha-like cracked yields. Although reference is made to hydrogenation, typically 40% or more of the starting VGO material is converted, i.e., hydrocracked, material boiling above about 340.degree. C. in the hydrogenation step.
U.S. Pat. No. 4,324,935, WERNICKE et al., relates to a similar process to WERNICKE et al., supra, which involves an improved hydrogenation step which results in high quality fractions, i.e., gasoline materials. The 200.degree. C.-340.degree. C. boiling range hydrogenated product is steam cracked and then recycled to the hydrogenation step, which again is more of a hydrocracking than an hydrogenation because of the severity of the conversion of the starting material.